1. Field of the Invention
The present invention relates to a scanner for scanning an original copy while it is manually moved over it.
2. Description of the Prior Art
A conventional scanner which scans the original copy while being manually moved over it uses a moving speed detector to equalize the length of the output picture in the subscanning direction to that of the original copy. The scanner constantly detects the moving speed in the subscanning direction with its moving speed detector, and controls the number of picture output lines per unit length in the subscanning direction according to the detected moving speed.
FIGS. 9 and 10 are schematic diagrams of the moving speed detector of a hand scanner according to the prior art and of the structure of an encoder gear used in that detector, respectively. As illustrated in FIG. 9, the conventional moving speed detector is provided with a platen roller 5 which, in close contact with the original copy, rotates with the movement of the hand scanner; an encoder gear 12 which is rotated by the rotation of the platen roller 5 via gears 11a, 11b and 11c; and a photo interrupter 13 arranged so as to sandwich part of the encoder gear 12.
The encoder 12, in which a plurality of slits 12A are cut as shown in FIG. 10, either transmits or interrupts light from the light emitting section of the photo interrupter 13 as the encoder rotates. A speed detecting circuit (not shown) detects the rotating speed of the platen roller 5 by counting the periods of output pulse signals from the photo interrupter 13, which generates with the rotation of the encoder gear 12.
This moving speed detector, however, involves a large number of parts because it uses a plurality of gears including the encoder, resulting in a large mechanism and a high cost. Moreover, as the rotation of the platen roller is transmitted to the encoder gear via several gears, the speed detection is subject to errors, especially when the detector is started or stopped on account of clearances between the gears.
Meanwhile, the Gazette of the Japanese Patent Laid-open No. 248367 of 1987 (filed on Apr. 22, 1986) discloses a more compactly designed scanner. This scanner has an image sensor 72 arranged over an original copy 71 as illustrated in FIG. 11. The image sensor 72, as it scans the original copy 71, is manually moved along the subject copy 71 to subscan it. A belt 73, on which black-and-white pattern is recorded as shown in FIG. 12, is wound around the image sensor 72 as if to wrap two-bit reading elements S1 and S2 in, provided at an end of the image sensor 72. The belt 73, having a rotating shaft supported by the image sensor 72, is in contact with the subject copy 71, and turns as the image sensor 72 moves for subscanning.
As the belt 73 turns with the subscanning movement of the image sensor 72, the reading elements S1 and S2 at the end of the image sensor 72 read the periods of variations of the black-and-white pattern, and thereby detect the quantity of the movement of the image sensor 72. This scanner enables the quantity of movement to be detected accurately by having the two-bit reading elements read two rows of black-and-white pattern which are out of phase with each other.
The scanner disclosed in the Gazette of the Japanese Patent Laid-open No. 248367 of 1987 involves the following problems.
First, as it detects the reading speed from the phase lag between the two rows of black-and-white pattern read by the two-bit reading elements S1 and S2 of the image sensor 72, the black-and-white pattern recorded on the belt requires accuracy to below 1/16 mm when the resolution of the image sensor is, for instance, 200 dpi.
Second, if the black-and-white pattern is smeared, the quantity of movement cannot be detected accurately, because errors will occur on at least one of the outputs of the two reading elements to make it impossible to detect the phase lag accurately. Therefore, the control of the reading period according to the output of the detection of movement quantity would not function normally. If the resolution is 400 dpi or more, the adverse effect of smear will be even greater.
Third, the reading period of the subject copy reading elements and that of the reading elements S1 and S2 for reading the black-and-white pattern are presumed to be the same. However, if the moving speed of the scanner becomes faster than the reading speed of the image sensor, the outputs of detection of the black-and-white pattern on the belt 13 by the two reading elements S1 and S2 will become incorrect, making it impossible to accurately perceive the movement quantity of the scanner and resulting in the problem of inability to recognize any unclearness of the read picture that may occur.